King tropism

Bonus for each square that we attack in the flank where the opponent
king is. Squares that we attack twice and are not protected by an enemy
pawn count double.

Passed STC:
http://tests.stockfishchess.org/tests/view/577dfca60ebc5972faa166b8
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 48373 W: 9832 L: 9481 D: 29060

And LTC:
http://tests.stockfishchess.org/tests/view/577e77870ebc5972faa166df
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 8881 W: 1408 L: 1255 D: 6218

Bench: 7577046

src/evaluate.cpp

@@ -491,6 +491,18 @@ namespace {
     const Bitboard TRank2BB = (Us == WHITE ? Rank2BB  : Rank7BB);
     const Bitboard TRank7BB = (Us == WHITE ? Rank7BB  : Rank2BB);
 
+    const Bitboard TheirCamp = (Us == WHITE ? Rank4BB | Rank5BB | Rank6BB | Rank7BB | Rank8BB
+                                            : Rank5BB | Rank4BB | Rank3BB | Rank2BB | Rank1BB);
+
+    const Bitboard QueenSide   = TheirCamp & (FileABB | FileBBB | FileCBB | FileDBB);
+    const Bitboard CenterFiles = TheirCamp & (FileCBB | FileDBB | FileEBB | FileFBB);
+    const Bitboard KingSide    = TheirCamp & (FileEBB | FileFBB | FileGBB | FileHBB);
+
+    const Bitboard KingFlank[FILE_NB] = {
+      QueenSide, QueenSide, QueenSide, CenterFiles,
+      CenterFiles, KingSide, KingSide, KingSide
+    };
+
     enum { Minor, Rook };
 
     Bitboard b, weak, defended, safeThreats;
@@ -558,6 +570,18 @@ namespace {
 
     score += ThreatByPawnPush * popcount(b);
 
+    // King tropism: firstly, find squares that we attack in the enemy king flank
+    b = ei.attackedBy[Us][ALL_PIECES] & KingFlank[file_of(pos.square<KING>(Them))];
+
+    // Secondly, add to the bitboard the squares which we attack twice in that flank
+    // but which are not protected by a enemy pawn. Note the trick to shift away the
+    // previous attack bits to the empty part of the bitboard.
+    b =  (b & ei.attackedBy2[Us] & ~ei.attackedBy[Them][PAWN])
+       | (Us == WHITE ? b >> 4 : b << 4);
+
+    // Count all these squares with a single popcount
+    score += make_score(7 * popcount(b), 0);
+
     if (DoTrace)
         Trace::add(THREAT, Us, score);